Clarify compiler errors when a type is unsupported by CLOGS

Move compile-type checks from SFINAE restrictions into static_asserts. This gives clear error message to the user when a type is not sortable/scannable by CLOGS.

@bmerry, the only possible drawback is if you are planning to add other versions of scan/sort algorithms, that would actually use the SFINAE checks for disambiguation. If that is not the case, I would like to merge this?